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          <th colspan="3" align="center">Chapter 5. Secondary Databases</th>
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      <div class="titlepage">
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          <div>
            <h2 class="title"><a id="indexes"></a>Chapter 5. Secondary Databases</h2>
          </div>
        </div>
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      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="indexes.html#CoreDbAssociate">Opening and Closing Secondary Databases</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="keyCreator.html">Implementing Key 
        
        <span>Extractors</span>
        </a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="keyCreator.html#multikeys">Working with Multiple Keys</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="readSecondary.html">Reading Secondary Databases</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="secondaryDelete.html">Deleting Secondary Database Records</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="secondaryCursor.html">
        
        <span>Using Cursors with Secondary Databases</span>
    </a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="joins.html">Database Joins</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="joins.html#joinUsage">Using Join Cursors</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="coreindexusage.html">Secondary Database Example</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="coreindexusage.html#edlWIndexes">Secondary Databases with example_database_load</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="coreindexusage.html#edrWIndexes">Secondary Databases with example_database_read</a>
                </span>
              </dt>
            </dl>
          </dd>
        </dl>
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      <p>
    Usually you find database records by means of the record's key.  However,
    the key that you use for your record will not always contain the
    information required to provide you with rapid access to the data that you
    want to retrieve. For example, suppose your 
        
        <span>database</span>
    contains records related to users. The key might be a string that is some
    unique identifier for the person, such as a user ID. Each record's data,
    however, would likely contain a complex object containing details about
    people such as names, addresses, phone numbers, and so forth.
    While your application may frequently want to query a person by user
    ID (that is, by the information stored in the key), it may also on occasion
    want to locate people by, say, their name.
  </p>
      <p>
    Rather than iterate through all of the records in your database, examining
    each in turn for a given person's name, you create indexes based on names
    and then just search that index for the name that you want.  You can do this
    using secondary databases. In DB, the 
        
        <span>database</span> 
    that contains your data is called a
    <span class="emphasis"><em>primary database</em></span>. A database that provides an
    alternative set of keys to access that data is called a <span class="emphasis"><em>secondary
    database</em></span><span>.</span> In a secondary database, the keys are your alternative 
    (or secondary) index, and the data corresponds to a primary record's key.
  </p>
      <p>
    You create a secondary database by creating the database, opening it, and
    then <span class="emphasis"><em>associating</em></span> the database with 
    the <span class="emphasis"><em>primary</em></span> database (that is, the database for which
    you are creating the index). As a part of associating
    the secondary database to the primary, you must provide a callback that is
    used to create the secondary database keys. Typically this callback creates
    a key based on data found in the primary database record's key or data.
  </p>
      <p>
  Once opened, DB manages secondary databases for you. Adding or deleting
  records in your primary database causes DB to update the secondary as
  necessary. Further, changing a record's data in the primary database may cause
  DB to modify a record in the secondary, depending on whether the change
  forces a modification of a key in the secondary database.
  </p>
      <p>
    Note that you can not write directly to a secondary database. 

    

    <span>Any attempt to write to a secondary database
    results in a non-zero status return.</span>
    
    To change the data referenced by a 
        
        <span>secondary</span>
    record, modify the primary database instead. The exception to this rule is
    that delete operations are allowed on the
         
        <span>secondary database.</span> 
    
    See <a class="xref" href="secondaryDelete.html" title="Deleting Secondary Database Records">Deleting Secondary Database Records</a> for more
    information.
  </p>
      <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
        <h3 class="title">Note</h3>
        <p>

        Secondary database records are updated/created by DB 
        only if the 
            
            <span>key creator callback function</span>
        returns 
             
            <span><code class="literal">0</code>.</span> 
        If 
             
            <span>a value other than <code class="literal">0</code></span>
        is returned, then DB will not add the key to the secondary database, and 
        in the event of a record update it will remove any existing key. 

        <span>Note that the callback can use either
        <code class="literal">DB_DONOTINDEX</code> or some error code outside of DB's
        name space to indicate that the entry should not be indexed.</span>

     </p>
        <p>
        See <a class="xref" href="keyCreator.html" title="Implementing Key Extractors">Implementing Key 
        
        <span>Extractors</span>
        </a> for more
            
            <span>information.</span>
    
    </p>
      </div>
      <p>
    When you read a record from a secondary database, DB automatically
    returns 
         
        <span>the data and optionally the key</span> 
    from the corresponding record in the primary database.
    
  </p>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="CoreDbAssociate"></a>Opening and Closing Secondary Databases</h2>
            </div>
          </div>
        </div>
        <p>
        You manage secondary database opens and closes in the same way as you
        would any normal database. The only difference is that:
    </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                You must associate the secondary to a primary database using
                    <span><code class="methodname">DB-&gt;associate()</code>.</span>
                    
            </p>
            </li>
            <li>
              <p>
                When closing your databases, it is a good idea to make sure you
                close your secondaries before closing your primaries. This is
                particularly true if your database closes are not single
                threaded.
            </p>
            </li>
          </ul>
        </div>
        <p>
        When you associate a secondary to a primary database, you must provide a
        callback that is used to generate the secondary's keys. These
        callbacks are described in the next section.
     </p>
        <p>
        For example, to open a secondary database and associate it to a primary
        database:
     </p>
        <a id="c_index1"></a>
        <pre class="programlisting">#include &lt;db.h&gt;

...

DB *dbp, *sdbp;    /* Primary and secondary DB handles */
u_int32_t flags;   /* Primary database open flags */
int ret;           /* Function return value */

/* Primary */
ret = db_create(&amp;dbp, NULL, 0);
if (ret != 0) {
  /* Error handling goes here */
}

/* Secondary */
ret = db_create(&amp;sdbp, NULL, 0);
if (ret != 0) {
  /* Error handling goes here */
}

/* Usually we want to support duplicates for secondary databases */
ret = sdbp-&gt;set_flags(sdbp, DB_DUPSORT);
if (ret != 0) {
  /* Error handling goes here */
}


/* Database open flags */
flags = DB_CREATE;    /* If the database does not exist,
                       * create it.*/

/* open the primary database */
ret = dbp-&gt;open(dbp,        /* DB structure pointer */
                NULL,       /* Transaction pointer */
                "my_db.db", /* On-disk file that holds the database. */
                NULL,       /* Optional logical database name */
                DB_BTREE,   /* Database access method */
                flags,      /* Open flags */
                0);         /* File mode (using defaults) */
if (ret != 0) {
  /* Error handling goes here */
}

/* open the secondary database */
ret = sdbp-&gt;open(sdbp,          /* DB structure pointer */
                 NULL,          /* Transaction pointer */
                 "my_secdb.db", /* On-disk file that holds the 
                                   database. */
                 NULL,          /* Optional logical database name */
                 DB_BTREE,      /* Database access method */
                 flags,         /* Open flags */
                 0);            /* File mode (using defaults) */
if (ret != 0) {
  /* Error handling goes here */
}

/* Now associate the secondary to the primary */
dbp-&gt;associate(dbp,            /* Primary database */
               NULL,           /* TXN id */
               sdbp,           /* Secondary database */
               get_sales_rep,  /* Callback used for key creation. Not
                                * defined in this example. See the next
                                * section. */
               0);              /* Flags */</pre>
        <p>
    Closing the primary and secondary databases is accomplished exactly as you
    would for any database:
  </p>
        <a id="c_index2"></a>
        <pre class="programlisting">/* Close the secondary before the primary */
if (sdbp != NULL)
    sdbp-&gt;close(sdbp, 0);
if (dbp != NULL)
    dbp-&gt;close(dbp, 0); </pre>
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